1. Field of the Invention
The invention relates to a method for preparing high wet-strength, increased crack-resistance, green bodies of cordierite-forming mixtures, to a method for preparing cordierite substrates and to the mixtures of raw materials and processing aids utilized to prepare such high wet-strength, increased crack-resistance green bodies and cordierite substrates.
2. Discussion of the Related Art
The exhaust gases emitted by internal combustion systems utilizing hydrocarbon fuels, such as hydrocarbon gases, gasoline or diesel fuel, can cause serious pollution of the atmosphere. Among the many pollutants in these exhaust gases are hydrocarbons and oxygen-containing compounds, the latter including nitrogen oxides (NOx) and carbon monoxide (CO). The automotive industry has for many years attempted to reduce the quantities of gaseous emissions from automobile engine systems, the first automobiles equipped with catalytic converters having been introduced in the mid 1970's.
Catalytic converters often comprise cordierite substrates, onto which are coated noble metal catalysts. The cordierite substrates arc typically in the form of a honeycomb body or multichannel monolith having substantially parallel cells or passages extending between open end faces thereof. The converter is placed in the path of the exhausteffluent of an automobile engine where the catalysts may act to convert hydrocarbons, CO and NOx to their non-toxic by-products, specifically water, carbon dioxide and reduced nitrogen species.
It is known to those skilled in the art, that a sintered cordierite structure is typically made by mixing raw materials such as talc, calcined and raw kaolin clay, including delaminated clay, alumina, silica, aluminum hydroxide, various inorganic and organic binders and water to form a batch composition. See, for example, the disclosure of U.S. Pat. No. 3,885,977. This composition is then extruded into a honeycomb body. At this stage, before sintering, the honeycomb body is frequently referred to as being "green". As such the term "green" as is used in the art and in this application refers to the state of a formed body or piece made of sinterable powder or particular material that has not yet been fired to the final, desired sintered ceramic product.
Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. The move toward producing thinner-walled cordierite monoliths or honeycombs, for use as catalyst carriers, has resulted in the following advantages: (1) honeycomb bodies exhibiting lower exhaust vehicle backpressure, which in turn results in an increase in engine power; (2) increased cell density honeycomb bodies which achieve better exhaust gas conversion with no increase in backpressure; and (3) thinner walled honeycomb bodies having less mass results in catalytic converters having faster and improved "light-off". It has also been desirable to optimize the thermal shock resistance and strength of the cordierite substrates. It has been suggested, in U.S. Pat. No. 4,772,580, (Hamanaka et al.) that these properties of cordierite structures can be enhanced by utilizing fine particles of both talc and kaolin in the preparation of the cordierite. Specifically, Hamanaka discloses the use talc particles having an average particle diameter of not greater than 7 .mu.m and kaolin particles having an average particle size of not greater than 2 .mu.m. More recently, it has been disclosed in U.S. Pat. No. 5,296,423 (Locker) that high wet and isostatic strength cordierite substrates can be prepared using about five to thirty percent delaminated kaolin coupled with fine particle size minerals of alumina and magnesia, less than 1 and 7 .mu.m, respectively.
Although the use of such fine raw materials leads to improved green and sintered thinwall honeycomb bodies having acceptable isostatic and wet strength, the demand honeycomb bodies having thinner and thinner cell walls continues. A recent processing innovation which has enabled the formation of thinner wall substrates approaching cell wall sizes of less than 6 mils is disclosed in U.S. Pat. No. 6,080,345 (Chalasani et al.). The process involves the use of a powder mixture, for forming honeycomb structures, that includes inorganic powder materials, binder, solvent for the binder, surfactant, and a component which is non-solvent with respect to the binder, solvent and inorganic powder materials. This powder mixture is mixed, plasticized and shaped to form a green ceramic preform body having improved wet strength and is thus especially suitable for use in the processing of thin walled honeycomb structures. Furthermore, Chalasani discloses a preferred aqueous binder system mixture that includes water, cellulose ether and a hydrophobic non-solvent.
While this Chalasani reference provides significant advances in the capability of the art to form complex, thin-walled ceramic honeycomb bodies through extrusion, the inclusion of this non-solvent in the powder, e.g., light mineral oil, results in additional complications in the "burnout" or removal of the binder. Specifically, difficulties are associated with the removal of the binder components from the shaped ceramic part without incurring distortion or breakage of the part. Specifically, because of the reduced strength of the thin-wall ceramic honeycomb bodies and the corresponding increase in the dimensional changes during binder removal due to the exothermic nature of the removal of the oil, special considerations in the firing of the ceramic honeycomb must be undertaken to avoid cracking of the ceramic body. Specially designed kilns, apparatus for volatile removal, reduced oxygen containing atmospheres and increased, complicated firing cycles are among the numerous means that have been employed to reduce the differential shrinkage and high cracking frequency experienced during the firing of thin-walled ceramic honeycomb bodies which incorporate the aforementioned binder.
There is, accordingly, a clear need for a means for preparing thin-walled cordierite structures which exhibit good wet strength in the green state and that are capable of being fired into a desired ceramic article without high differential shrinkage and incidences of cracking or defects.